A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to store compressed air and to use the stored, compressed air to supplement the air charge from the turbocharger compressor. For example, Pursifull et al. describe a system in US 2011/0132335 wherein compressed air is stored in a boost reservoir and is dispensed into the intake manifold when insufficient compressed air is available from the turbocharger compressor. In particular, the boost reservoir is charged with fresh intake air and/or effluent from one or more unfueled cylinders. By dispensing extra compressed air from the boost reservoir to the intake manifold, torque corresponding to the dispensed air can be provided to meet the torque demand while the turbine spins up.
However, the inventors herein have identified potential issues with such a system. As one example, if the boost reservoir has a small volume, the boost air may be initially able to supply sufficient air to provide increased desired torque, but after the supply is depleted, such as at higher engine speeds, the turbine may still not be spun up, and thus torque may drop following the initial increase. Such performance may be worse than no compensation at all. Further still, the pressure of the air dispensed by the boost reservoir may not be sufficiently high to overcome the boost pressure. As a result, even with the discharged boost air, turbo lag may not be sufficiently addressed.
Thus, at least some of the above issues may be addressed by a method for a turbocharged engine. In one embodiment, the method comprises, during a tip-in, operating a turbocharger compressor, discharging compressed air from a boost reservoir to an intake manifold, downstream of an intake throttle, for a duration, with the throttle closed. Then, after the duration, directing compressed air from the compressor to the intake manifold with intake throttle open. In this way, the throttle can be closed to allow boost air from a compressor to be pre-charged upstream of the throttle while boost air is discharged from the boost reservoir downstream of the throttle.
For example, during a tip-in, when a throttle inlet pressure (TIP) is below a threshold, the compressor may be operated with the throttle closed to raise TIP while high pressure boost air is discharged downstream of the closed throttle to meet the torque demand. Then, once TIP is above the threshold, the throttle may be opened and pre-charged high pressure boost air from the compressor can be discharged from upstream of the throttle into the engine to meet the torque demand. As such, by pre-charging the compressor boost air with the throttle closed, TIP may be raised faster than would be otherwise possible. By allowing boost air from the reservoir to be discharged into the intake while a pressure of the compressor boost air is raised, turbo lag may be better addressed while also meeting the torque demands. Overall, boosted engine performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.